I. The Incomprehensibility and Knowability of the Divine Essence
"God cannot be defined, for the reason that he is immense, and because we are ignorant of his essence." [Zacharias Ursinus - Commentary on the Heidelberg]
"His essence, indeed, is incomprehensible, utterly transcending all human thought; but on each of his works his glory is engraven in characters so bright, so distinct, and so illustrious, that none, however dull and illiterate, can plead ignorance as their excuse." "the most direct path and the fittest method is, not to attempt with presumptuous curiosity to pry into his essence, which is rather to be adored than minutely discussed, but to contemplate him in his works, by which he draws near, becomes familiar, and in a manner communicates himself to us." [John Calvin - Institutes]
"Already immediately when it makes a beginning, it stands before the Incomprehensible. From Him it takes its beginning, for out of Him are all things. But also in the further loci, when it descends to the creatures, it considers these no otherwise than in their relation to God, as they are out of and through and unto Him. Thus then the knowledge of God is the only dogma, the exclusive content of the whole dogmatics."
"There is no knowledge of God as He is in Himself. We are human beings and He is the Lord our God. There is no name that fully expresses his being; no definition that circumscribes Him. He is infinitely far exalted above our imagination, above our thought, above our language." [Herman Bavinck - Reformed Dogmatics, Vol. 2]
"God, therefore, is in his nature a substance, or essence, which is infinite, eternal, and unchangeable; the common subject of all divine perfections, and the common agent of all divine acts." [Charles Hodge – Systematic Theology]
"it is impossible for any one to give a perfect description of God; since he is incomprehensible. No words can fully express, or set forth, his perfections." [Thomas Ridgley - WLC - A Body of Divinity]
II. The Essential Names and Theological Descriptions of God
"God is a spiritual essence, intelligent, eternal, different from all creatures, incomprehensible, most perfect in himself, immutable, of immense power, wisdom and goodness; just, true, pure, merciful, bountiful, most free, hating sin—which is, the eternal Father, who from eternity begat the Son in his own image; the Son, who is the co-eternal image of the Father; and the Holy Ghost, proceeding from the Father and the Son, as has been divinely revealed by the sure word delivered by the Prophets and Apostles, and divine testimonies..." [Zacharias Ursinus - Commentary on the Heidelberg]
"God is a spirit." "God is infinite, eternal, and unchangeable, in his being, wisdom, power, holiness, justice, goodness, and truth" [Thomas Vincent - The Shorter Catechism]
"Thus in Scripture He is called El, the Strong One, Elschaddai the Mighty One, Jahveh, the Being One, further Father, Son, Spirit, good, merciful, gracious, righteous, holy etc., all concepts that first apply to creatures and then are transferred to God in an eminent sense." [Herman Bavinck - Reformed Dogmatics, Vol. 2]
"God is a Spirit, in and of himself, infinite in being, glory, blessedness, and perfection, all sufficient, eternal, unchangeable, incomprehensible, every where present, almighty, knowing all things, most wise, most holy, most just, most merciful and gracious, long-suffering, and abundant in goodness and truth." [Thomas Ridgley - WLC - A Body of Divinity]
III. The Oneness and Unicity of God (Unitas Singularitatis)
"The unity of God is proven, in the first place, by the express testimony of Scripture. "Hear, O Israel, the Lord our God, is one God." "See now that I, even I, am he, and there is no God with me." "I am the First and the Last, and beside me there is no God." "We know that an idol is nothing in the world, and that there is none other God but one." "There is one God, and one Mediator between God and man, the man Christ Jesus."" [Zacharias Ursinus - Commentary on the Heidelberg]
"the Scripture, in order to direct us to the true God, distinctly excludes and rejects all the gods of the heathen, because religion was universally adulterated in almost every age." [John Calvin - Institutes]
"The God of revelation is no abstract unity, but the living, true God, who in the infinite fullness of his life includes the highest diversity." [Herman Bavinck - Reformed Dogmatics, Vol. 2]
"The unitas singularitatis. This attribute stresses both the oneness and the unicity of God, the fact that He is numerically one and that as such He is unique. It implies that there is but one Divine Being, that from the nature of the case there can be but one, and that all other beings exist of and through and unto Him." [Louis Berkhof - Systematic Theology]
"There is but one only, the living and true God." [Thomas Vincent - The Shorter Catechism]
"The Lord Jehovah (one God in three people) is the true, living, eternal God; and him we must have for our God." [Thomas Watson - The Ten Commandments (Modernized)]
"Scripture is very express in asserting the unity of the Godhead. It is said, 'The Lord our God is one Lord;' and 'I, even I, am he; and there is no God with me;' and 'The Lord, he is God; there is none else besides him;' and elsewhere, 'Thou art God alone.'" [Thomas Ridgley - WLC - A Body of Divinity]
IV. Rational Arguments and Proofs of the Divine Unity
"He who alone reigns over all, and governs all things in the same way, and so possesses supreme power and majesty, cannot be more than one. But there is no one, beside God, who is so supreme and great, that no greater can either exist or be conceived of."
"There cannot be more than one being that is omnipotent, for if there were many, they would mutually hinder and oppose each other, and so would not be omnipotent." [Zacharias Ursinus - Commentary on the Heidelberg]
"There cannot be more than one being that is infinite, or immense; for if there were more than one, no one would be everywhere. Hence, there cannot be many Gods, but only one God, who alone is infinite."
"There can be but one first cause of all things. God is that first cause. Therefore, he is one God, excluding all others." [Zacharias Ursinus - Commentary on the Heidelberg]
"There is but one Omnipotent Power. If there are two omnipotent, we must always suppose a contest between the two: that which one would do, the other, being equal, would oppose; and so all things would be brought into confusion." [Thomas Watson - The Ten Commandments (Modernized)]
"There is but one independent being, who is in and of himself, and derives his perfections from no other; and therefore there can be but one God." "As it implies that divine perfection is boundless, so it denotes that he sets bounds to the perfections of all others. If, therefore, there are other gods, their perfections must be limited; but that which is not infinite, is not God." [Thomas Ridgley - WLC - A Body of Divinity]
"God is infinite, and there cannot be more than one infinite Being, forasmuch as one infinite Being doth set bounds and limits unto all other beings, and nothing that is bounded and limited can be infinite." [Thomas Vincent - The Shorter Catechism]
V. The Simplicity and Qualitative Oneness of the Essence (Unitas Simplicitatis)
"The unitas simplicitatis. While the unity discussed in the preceding sets God apart from other beings, the perfection now under consideration is expressive of the inner and qualitative unity of the Divine Being. When we speak of the simplicity of God, we use the term to describe the state or quality of being simple, the condition of being free from division into parts, and therefore from compositeness. It means that God is not composite and is not susceptible of division in any sense of the word." [Louis Berkhof - Systematic Theology]
"By the term, Essence, we are to understand, in reference to this subject, that which the eternal Father, Son, and Holy Ghost are considered, and declared to be, singly and absolutely in themselves, and which is common to the three. By the term, Person, however, we are to understand that which the three persons of the Godhead are considered and declared to be individually and relatively, or as compared with each other, and which they are according to the mode of existence peculiar to each." "the persons do not constitute something abstracted or separated from the essence which they have in common, nor that the essence is any fourth thing separate from the three persons; but each of them is the entire and self-same essence of the Divinity. But the difference consists in this, that the persons are each distinct from the other, whilst the essence is common to the three." [Zacharias Ursinus - Commentary on the Heidelberg]
"While he proclaims his unity, he distinctly sets it before us as existing in three persons. These we must hold, unless the bare and empty name of Deity merely is to flutter in our brain without any genuine knowledge." "the essence of God being simple and undivided, and contained in himself entire, in full perfection, without partition or diminution"
"When we profess to believe in one God, by the name God is understood the one simple essence, comprehending three persons or hypostases; and, accordingly, whenever the name of God is used indefinitely, the Son and Spirit, not less than the Father, is meant." [John Calvin - Institutes]
"By the doctrine of the simplicitas Dei, Christian theology was after all safeguarded against the error of assuming an essential difference between the two." "Every attribute is God’s essence. God is what he has. In creatures there is a distinction between what they are and what they have; a human being is still a human being, even if he has lost the image of God and has become a sinner. But in God all attributes are his essence itself." [Herman Bavinck - Reformed Dogmatics, Vol. 2]
"As we are not to assert a plurality of gods, so we are not to think or speak of God in such a way as tends to overthrow the simplicity of the divine nature." "When we speak of the divine perfections, as being in God, we suppose them all essential to him, as opposed to what is accidental. An accident is generally described as what belongs, or is superadded, to a being or subject..."
"But this is by no means to be affirmed of the divine being and attributes; for to suppose God to be destitute of any of them, is as much as to say that he is not infinitely perfect, or that he is not God." [Thomas Ridgley - WLC - A Body of Divinity]